1. Field of the Invention
The present invention relates generally to catalyst compositions and, more particularly, the invention relates to a catalyst useful in the conversion of mercaptans to disulfides in petroleum distillate chargestocks.
2. Description of Related Art
The removal of mercaptan sulfur from hydrocarbons, particularly petroleum distillates, such as cracked gasolines, straight gasolines, etc., is an ongoing problem. Mercaptans are objectionable due to their corrosive nature and their strong odor.
A well known and generally accepted process for treating hydrocarbon fractions to convert mercaptans to less objectionable disulfides by catalytic oxidation is the UOP Merox.RTM. process. In one form, the Merox.RTM. process uses a catalyst which is soluble in caustic solution in liquid-liquid contact with the hydrocarbon fractions.
The type of catalyst most widely used in the Merox.RTM. process and similar types of mercaptan oxidation processes include sulfonated derivatives of metal phthalocyanine compounds. Cobalt phthalocyanine sulfonates are preferred.
The preparation and use of metal phthalocyanine sulfonates having mono-, di-, tri-, and tetra sulfonate functionality is well described in the literature. See, for example, U.S. Pat. Nos. 3,039,855 (Urban), 3,108,081 (Gleim), 4,003,827 (Carlson, et al.), and 4,049,572 (Douglas), all assigned to Universal Oil Products Company, and Fukada, in Nippon Kagoku Zasshi, Volume 79, at pages 396-399 (1958). Each of the foregoing patents and publication is incorporated herein by reference.
Metal phthalocyanine sulfonates, in particular cobalt phthalocyanine sulfonates, differ in activity and in their solubility characteristics depending at least in part on their respective sulfonate functionalities. This has caused a number of problems for users of these materials.
For example, cobalt phthalocyanine disulfonate, commonly used in mercaptan oxidation units calling for two phase liquid-liquid contact, was at one time sold in a dry powdered form which was extremely dusty. This was undesirable because cobalt phthalocyanine sulfonates are strong dyes. Several different approaches were attempted in order to eliminate handling problems, including the packaging of disulfonate material in water soluble bags; mixing the material with an antidusting agent such as PTFE; and admixing the disulfonate compound with water and selling the product as a slurry.
Although the packaging of cobalt phthalocyanine disulfonate as an aqueous slurry is widely accepted, this approach suffers from several problems. With insufficient mixing, the disulfonate material precipitates from the slurry. If the slurry is mixed sufficiently to retain the disulfonate in suspension for acceptable lengths of time, the slurry becomes extremely viscous and may gel, making it very difficult to remove the material from packaging.
The use of cobalt phthalocyanine tetrasulfonate, which is highly soluble in water, can eliminate the precipitation and gelation problems associated with the use of disulfonate slurries. However, reports of actual operating experience in the field and laboratory tests indicate that the catalytic activity of the tetrasulfonate form of cobalt phthalocyanine is not as high as that of the disulfonate form.